The present invention relates to an information recording medium on/from which information is recorded/reproduced in units of sectors and, more particularly, to an information recording medium such as an optical disk for recording/reproducing information using a change in optical characteristics due to a phase change, i.e., the difference in reflectance between the crystalline state and amorphous state of a recording film due to a change in optical constant, and a method of recording information in this information recording medium.
Examples of so-called rewritable optical disks allowing information recording/reproduction, which are already commercially available, are a 120-mm magneto-optical disk, a 90-mm magneto-optical disk, and a 120-mm phase change disk (so-called PD).
Of these optical disks, the phase change optical disk records information by irradiating a laser beam whose irradiation intensity is modulated in correspondence with the information to be recorded. This phase change optical disk does not require a means for generating a magnetic field on the recording apparatus side, unlike an optomagnetic disk, and hence is advantageous to reduce the size and weight of the recording apparatus.
For the phase change optical disk, a laser beam is irradiated on the recording layer (recording film) to record information. When the recording layer portion in a crystalline state is heated to the melting point or more and then abruptly cooled, the crystalline state changes to an amorphous state. When the recording layer portion in the amorphous state is heated to a predetermined temperature equal to or lower than the melting point and then cooled, the amorphous state changes to the crystalline state.
To record information on the phase change optical disk having a recording layer with such characteristics, the recording layer on which information is to be recorded is heated to the melting point or more by irradiating the laser beam. Upon abruptly cooling the recording layer immediately after laser beam irradiation, the recording layer which had been in the crystalline state before laser beam irradiation phase-changes to the amorphous structure.
The most disadvantageous point of the phase change optical disk is degradation in phase change recording layer due to a repeated overwrite, i.e., a data rewrite. In an overwrite, the temperature of the phase change recording layer of the phase change optical disk is increased to the melting point or more and then abruptly cooled to form the amorphous. At this time, the phase change recording layer is temporarily melted. For this reason, when one portion is repeatedly subjected to the overwrite, the phase change recording layer moves along the scanning direction of laser beam (this phenomenon will be referred to as substance movement hereinafter). If the overwrite is repeated, the substance movement is accelerated to result in the omission of the phase change recording layer itself depending on the position, so data may be damaged. Especially, the waveform may be largely distorted at the initial defective portion and the start and end portions of data. This is because substance movement is conspicuous in the recording layer area adjacent to a normally solid area where no data is written.
If information is repeatedly recorded on one sector of the phase change optical disk tens of hundreds of times using laser beam radiation, the substance movement degrades the signal quality over the recording area of this sector. This will be referred to as degradation (whole field degradation) due to repeated recording. This degradation in signal quality is particularly conspicuous at the start and end portions of the recording portion, as described above. More specifically, since the conventional recording apparatus starts to write data from a predetermined portion, not only the whole field degradation but also the substance movement is especially conspicuous at the recording start and end portions of the recording area. This results in a larger distortion in signal than that due to whole field degradation, so signal reproduction becomes difficult. This will be referred to as start/end portion degradation due to repeated recording.